A Practical Guide To Rotor Dynamics

by Malcolm E. Leader, P.E.


This is an excellent paper for those wanting to understand the fundamentals of rotor dynamics. There are a number of illustrations, graphs, and charts that are used to describe the subject material and the effects various conditions have on rotor dynamics. Not all condition changes lead to an intuitive change in the rotor dynamic results and the illustrations and charts provide a clear description of each variable’s effect on a rotor’s dynamics. This is a very practical paper providing both rule of thumb guidance and the fundamental equations used to describe rotor dynamics. There are a number of equations for the beginner and intermediate analysts to further their understanding of the subject and they are used with well thought out descriptions of the subject matter. This paper is not a case history style document, but does use actual rotor calculations to work through the subject specifics. There is also a nomenclature page which documents the variables used within the paper to ensure clarity of the equations and descriptions. This is a well written introduction to Rotor Dynamics that is often a confusing and difficult subject for many beginning analysts.


Rotor dynamics is a very interesting and complicated subject. The importance of this subject has increased over the last few decades as machine speeds have increased and higher flows and efficiencies have had the side effect of introducing problems with critical speeds, unbalance response and rotor stability. Some of these problems are related to the economics of the capital expenditures where a machine may be bought on a cost basis. Far too often this results in excessive and expensive rebuilds and costly lost production. This paper will introduce the basics of rotor dynamics and lead into some of the more advanced concepts. The mathematics will be kept to a minimum and as many helpful “rules of thumb” will be included as this subject allows. Units are shown in detail for each equation.

At times, rotor dynamics can be a very controversial subject from the nomenclature used to the question of the degree of accuracy needed to model a rotor dynamic system. There have been many simplifications and assumptions made in this paper and the author offers some opinions that some people may disagree with, but the approach here is quite conservative and has proven successful over the last 25 years of my career. The approaches and guidance offered here are based on experience of many people and many years of
analyzing and testing machinery. Analytical and actual test data will be used as examples.

A nomenclature page is included and whenever possible, the symbols used for various factors are identified in the body of the paper. At the end of the paper is a list of references that the author has found very valuable and are recommended for further reading.

Rules of thumb are fine but the reader should come to the conclusion that the only way to do a meaningful rotor dynamics analysis on a machine train will be to procure the necessary computer programs to do the analysis. Some sources for programs used in the preparation of this paper are noted in the acknowledgments. There are many sources for computer programs that will run well on personal computers. While some are easier to use than others and some use differing analytical techniques, like any tool, these programs are dependent on the skills of the user. GIGO remains a problem today and the more powerful the program, the more pitfalls there are waiting. Thus, the main purposes of this paper are to introduce the basic concepts of rotor dynamics and to serve as a reality check guide.”

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